Thanks to the Kepler planet-hunting mission and other surveys, the number of potential exoplanets—planets orbiting other stars—has grown. Of the 1,235 possible planets in the 2011 Kepler catalog, over half are smaller in size than Neptune. This indicates at least some may be rocky, similar in some respects to the terrestrial planets in the Solar System. While confirming and characterizing the individual planets is ongoing work, enough candidates are available that astronomers can perform statistical analyses on the worlds and their host star systems.

In particular, a new study published in Nature examined the relationship between the host star's chemistry and the size of the planets in orbit. The presence of chemical elements heavier than helium—which astronomers perversely refer to as metals—in a star's spectrum is a measure of the environment of planet formation. As described by authors Lars A. Buchhave et al., no strong correlation exists between the metal content of the host star and the presence of low-mass planets. This is in stark contrast to higher-mass planets (comparable to Jupiter), which preferentially orbit high-metal stars. In other words, terrestrial planets may orbit a higher fraction of stars in the galaxy, since they don't require a metal-rich environment for formation.

The widely accepted model of planet formation involves a disk of gas and dust surrounding the newborn star, known as the protoplanetary disk. As the name suggests, the protoplanetary disk gives rise to planets, but it also deposits material onto the star. The chemical composition of the material becomes part of the star's spectrum, which can be measured. The relative abundance of metals (compared to the base amount of hydrogen, which comprises most of any star's makeup) is known as the metallicity.

While the fraction of metals in any star is less than 1 percent by mass, modern astronomical methods are able to distinguish small variations from star to star. Additionally, the researchers in the current study developed a new statistical tool that allowed them to characterize metallicities even when the data was noisy, as with the Kepler catalog. (Noise in this case refers to excess photons from other sources; follow-up observations can reduce this noise by looking at the target star for longer times). Their technique involved matching real stellar spectra to simulated spectra with comparable noise levels.

Kepler identifies exoplanet candidates when they pass in front of their host stars, relative to observers on Earth. The amount of light blocked during the transit yields the planet's size (similar to how astronomers measured the size of the Solar System during the transits of Venus in the 18th and 19th centuries). Initial observations identify potential exoplanets, and follow-up observations are required to confirm whether they are actually planets. The advantage of this type of observation is that it can ideally locate planets that are smaller and less massive than what is found in other methods.

The authors examined 226 exoplanet candidates orbiting 152 stars. Of these, 175 potential planets were smaller than Neptune, which itself is about four times the diameter of Earth. Due to the limitations inherent in all planet-hunting methods, most of these candidates orbited very close to their host stars (about the distance of Mercury's orbit or closer). Some of the candidates were comparable to the smallest exoplanets yet discovered, though the errors preclude clear identification of Earth-sized planets at this time. The researchers compared the metallicity of the host stars to the radius of the exoplanet candidates.

While larger planets (those with radii bigger than four times Earth's) were strongly correlated with high-metallicity stars, the researchers found no clear relationship for small planets. Worlds smaller than Neptune were as likely to be found orbiting low-metallicity stars as high, meaning the protoplanetary environment giving them birth was far more diverse than for larger planets.

According to a widely accepted theory of planet formation, the gas in protoplanetary disks dissipates relatively quickly. As the authors point out, if this theory is correct, then a higher metallicity would increase the mass of protoplanetary cores, since the raw materials would contain a higher fraction of heavier chemical elements. Heavier grains of dust in high metallicity systems slowed the dissipation rate, in other words, meaning high-mass planets would have time to form. Low-mass planets, on the other hand, don't have to race the dissipation clock. These can form even when the metallicity is relatively low.

While there is likely a lower limit to the metallicity possible for planet formation, the current study suggests it's much lower than other models previously concluded. This means small, terrestrial planets could be very common in the Milky Way, since they need far less restrictive conditions for their formation.

12 Reader Comments

Is there any consideration of planet density in this, or is it purely mass?----Offtopic:

What happened to the technopedia? I don't know how I missed the definition of exoplanet in the article (even using "find") on my first readthrough, but looking for it reminded me that I haven't seen the technopedia in a while.

This is in stark contrast to higher-mass planets (comparable to Jupiter), which preferentially orbit high-metal stars. In other words, terrestrial planets may orbit a higher fraction of stars in the galaxy, since they don't require a metal-rich environment for formation.

So what's the metal point of the sun in our own solar system ? Seeing as how it has a handful of rocky planets AND a few gas giants.

It makes sense that there would most likely be many times more smaller planets than larger ones in the Galaxy. If you look at the numbers of objects in our own solar system by size (counting all the moons and dwarf planets) you find that there are far more objects in the smaller ranges than in the larger. In fact some estimates say their could be trillions of objects in the Oort Cloud with a greater than 1 kilometer radius. But of course their compositions aren't necessarily "terrestrial" I guess.

Of the objects that are definitely terrestrial I think you can count the 4 inner planets, Earth's moon, Mars's moons, and the asteroid belt. I'm uncertain if you could define the objects that make up the various moon systems of the gas giants as terrestrial though.

The amount of light blocked during the transit yields the planet's size (similar to how astronomers measured the size of the Solar System during the transits of Venus in the 18th and 19th centuries).

This is incorrect.

The measurements for the Solar System's size used the timing for when Venus touched the edges of the Sun's disk, as seen from various places on Earth. Nothing could be learned about the solar system's size by the amount of light blocked by Venus since we had then no way to independently measure the size of the Sun or Venus.

My thoughts are as the solar system travels through the milky way there is constant drag making a disk system and rotating. The denser the material, the more energy it takes to change course and the drag will not pull it towards the sun. Close to the sun the material will go towards the sun leaving rocky planets. As the material gets farther out it looses much of the gravity pulling towards the middle the drag changes it a small amount but the inertia it has going around the solar system keeps it from going towards the sun.

In a high mass system the inertial energies are higher, as the planets coalesce there will be larger planets close to the star are able to keep the lighter gasses.

Is there any consideration of planet density in this, or is it purely mass?

I'd also be interested in planetary density - the article seems to suggest that a cloud with a higher proportion of heavier elements is more likely to lead to large planets - e.g. gas giants - made up mainly of the lighter elements? And that a cloud without a high percentage of heavy elements leads to more small, rocky plants e.g. made of heavy elements?I can see that a cloud of light elements would disperse faster than a cloud of heavy elements, but not how a cloud of heavy elements leads to more gas giants specifically?Also +1 I-ku-u, beat me to it!What I'd really be interested in is the amount of irregular transits that Keppler detects - how much non-luminous lumpy stuff is there floating around the universe that isn't orbiting a star?

Is there any consideration of planet density in this, or is it purely mass?

Kepler can only tell you the size of a candidate planet. Other measurements are done to turn 'candidate' into 'probably actual', and also determine the mass. Which gives you the density, and gives you a general sense of the composition (rocky, icy, gaseous).

Though the general assumption seems to be that worlds that are too small would necessarily be rocky or icy, because a small gaseous world would have low mass and the gas would just be blown away by the star or just evaporate due to temperature (particularly at the close orbits these exoplanets are in). Gas giants have enough gravity to hold on to their atmospheres.

This is in stark contrast to higher-mass planets (comparable to Jupiter), which preferentially orbit high-metal stars. In other words, terrestrial planets may orbit a higher fraction of stars in the galaxy, since they don't require a metal-rich environment for formation.

So what's the metal point of the sun in our own solar system ? Seeing as how it has a handful of rocky planets AND a few gas giants.

I'd assume other factors would play into this as well.

Red Giants - dwarf stars - larger / smaller Yellow stars - etc...

Stars change over time. Our Sun is a main sequence star, which will eventually turn into red giant.Our Solar System contains heavy elements - heavier than iron, which means that it formed from remnants of supernova. Stars cannot produce elements heavier than iron.

I'd say that remnants of died stars are more important factor for forming metal-rich planets than current state of host star.

Actually, Neptunes seems to be the most common, giants least common of the basic classes.

The habitable zone, where surface liquid water is possible, seems fairly well populated but they need more Kepler data for larger stars (longer orbits). In fact, about every system may have a superEarth in the habitable zone, but giants may populate some tens of percent of systems.

A fixed axis is not a habitability requirement if planets are well spaced, recent modeling implies large tilts will still mean habitability as tilt may be semi-stable over 500 million years. (Old models were wrong.) In fact, changing tilts can potentially help smaller terrestrial biospheres, seems Mars somewhat chaotic tilt (Jupiter) has made snowmelt surface water from time to time allowing eventual extant crustal life surface access.

"Earth like" is not well defined. Earth analogs are comparably sized terrestrials in the habitable zone of comparably sized stars. They are estimated to populate some ~ 1 - 2 % of systems (IIRC).

So what's the metal point of the sun in our own solar system ? Seeing as how it has a handful of rocky planets AND a few gas giants.

I'd assume other factors would play into this as well.

Red Giants - dwarf stars - larger / smaller Yellow stars - etc...

The Sun is fairly typical Milky Way average as per IBEX oxygen and neon data. It is however more metallic than the typical stars of our galactic neighborhood out here, probably because of its birth in a molecular cloud seeded by a supernova (as seen by isotope ratios).

Smaller stars have more terrestrials, so the plentiful M stars will have the most planets as a group.

Nonapod wrote:

Of the objects that are definitely terrestrial I think you can count the 4 inner planets, Earth's moon, Mars's moons, and the asteroid belt.

The term is usually retained for planets (planets, exoplanets, dwarf planets, protoplanets).

Mydrrin wrote:

My thoughts are as the solar system travels through the milky way there is constant drag making a disk system and rotating.

Planetary system formation is not dependent on the galactic neighborhood, which can be seen in how the stars and so systems rotation axis are randomly distributed.

It is an old, well understood, fact that when the gas nebula contracts the cloud parts angular momentum will cancel leaving a randomly oriented excess for the disk it collapses into.

justageezer wrote:

I can see that a cloud of light elements would disperse faster than a cloud of heavy elements, but not how a cloud of heavy elements leads to more gas giants specifically?

As the article says, more heavy elements means more massive cores (more dense). Being more massive they can gather more volatiles (gas) faster.

justageezer wrote:

What I'd really be interested in is the amount of irregular transits that Keppler detects - how much non-luminous lumpy stuff is there floating around the universe that isn't orbiting a star?

But microlensing (stars bending light) has shown as many nomad planets as stars. Models comply, most systems will eject one planet or more,* and maybe also capture temporarily. It is a lively neighborhood out there!

------------------* The best, latest Nice models of our own system predicts it has ejected one Neptune during formation. Again our system shows many "typical" characteristics, even if all systems looks to be individual.

So... what are the metallicities of Alpha Centauri A & B compared to Sol or other systems where we've supposedly detected "super earths"? Sure Proxima is technically closer, but since it's such a low energy, low mass star (and a flare to boot) probably not much point looking at it unless we find it has an asteroid belt full of little habitable bodies... wouldn't THAT be something?